In a rotary wing aircraft of the helicopter type, the pilot's flight controls enable the pilot to control the main lift and propulsion rotor.
Thus, a collective pitch lever and a cyclic stick act on servo-controls suitable for modifying the pitch of the blades of the main rotor. For example, a lightweight helicopter generally has three servo-controls, one servo-control for pitch control referred to for convenience as the “longitudinal servo-control”, and two servo-controls for rolling to left and to right to enable the pilot to control roll.
When the pilot operates the collective pitch lever, that delivers an order to vary the pitch of the blades of the main rotor collectively by the same amount, so as to adjust the lift provided by the blades. All three servo-controls are then operated simultaneously.
In contrast, the pilot controls pitching and roll of the helicopter by inclining the rotor disk that is described by the blades of the main rotor, by operating the cyclic stick. For example, by pushing the cyclic stick forwards, the pilot causes only the longitudinal servo-control to retract. Three linkages are then controlled by the cyclic stick, one longitudinal pitch linkage, one left roll linkage, and one right roll linkage, the linkages connecting the cyclic stick respectively to the longitudinal servo-control and to the left and right servo-controls, with the left and right roll linkages depending on each other.
Under such conditions, the collective pitch lever and the cyclic stick are connected mechanically to the servo-controls by a combiner, said “combiner” being the mechanical member where the cyclic pitch control and the collective pitch control are superposed.
The combiner then allows the cyclic pitch control and the collective pitch control to operate independently of each other and in theory without mutual interaction. The combiner forwards a collective pitch order to servo-controls when it is actuated by the collective pitch lever and a cyclic pitch order when it is actuated by the cyclic stick.
Such a combiner is conventionally provided with L-shaped crank means, each provided with first and second branches extending substantially at right angles to each other, each crank means being suitable for pivoting about a pivot point situated at the junction between the first and second branches.
For example, the combiner has first crank means connected mechanically via its first branch to the collective pitch lever and second crank means connected mechanically via its first branch to the cyclic stick via the pitch linkage. The second branch of the first crank means is then connected to the pivot point of the second crank means, while the second branch of the second crank means is connected to the longitudinal servo-control.
Such a combiner performs its function well. Nevertheless, in the event of the linkages moving through considerable distances, its accuracy suffers.
Document U.S. Pat. No. 4,482,115 describes another device provided with first crank means that are L-shaped and connected to second crank means that are T-shaped.
In addition, on a heavy rotary wing aircraft, it is essential for the servo-controls to be distributed equidistantly around a perimeter for mechanical strength reasons.
Consequently, the servo-controls are no longer on the natural pitch and roll axes. This new situation makes it necessary to combine the orders coming from the pitch and roll linkages.
Consequently, a heavy helicopter includes a phasing unit or “anticipator” upstream from the combiner. Reference may be made to the literature to obtain more information concerning such a phasing unit.
Furthermore in a rotary wing aircraft that is provided with first and second propulsive propellers disposed on either side of the fuselage, such as a hybrid helicopter as described in patent applications FR 2 916 418, FR 2 916 419, FR 2 916 420, FR 2 916 421, the anti-torque function is performed by the first and second propellers and not by a tail rotor.
Thus, the pilot controls the first and second pitches of the blades of the first and second propellers:                collectively to adjust the resultant thrust from the first and second propellers, using a thrust control lever, for example; and        differentially to control the aircraft in yaw, e.g. using a rudder bar.        
As for the main rotor, it is then appropriate to implement a device for combining the collective pitch control orders and the differential pitch control orders, e.g. a combiner.
Furthermore, if the rotary wing aircraft is provided with additional airfoils such as a vertical tail fin, it is advantageous to diminish the differential pitch of the blades of the first and second propellers as a function of an increase in air speed. The additional airfoils act at high speed to perform an anti-torque function suitable for countering part of the torque exerted on the fuselage by the main rotor, with the effectiveness of that action being proportional to the square of the forward speed.
Consequently, it is advantageous to couple the collective pitch control for the propellers with the differential pitch control. More precisely, means may be implemented so that a collective pitch control order has an influence on the differential pitch of the first and second propellers.